Many rubber articles, principally automobile tires, hoses, belts and the like are known as composites and are reinforced with fibers in cord form. In all such instances, the fiber must be firmly bonded to the rubber.
A frequent problem in making a rubber composite is maintaining good adhesion between the rubber and the reinforcement. A conventional method in promoting the adhesion between the rubber and the reinforcement is to pretreat the reinforcing fiber with a mixture of a rubber latex and a phenol-formaldehyde condensation product wherein the phenol is almost always resorcinol. This is the so called "RFL" (resorcinol-formaldehyde-latex) method. An alternative method of promoting such adhesion is to generate the resin in-situ (in the vulcanized rubber/textile matrix) by compounding a vulcanizing rubber stock composition with the phenol/formaldehyde condensation product (hereinafter referred to as the "in-situ method"). The components of the condensation product consist of a methylene acceptor and a methylene donor. The most common methylene donors include N-(substituted oxymethyl) melamine, hexamethylenetetramine or hexamethoxymethylmelamine. A common methylene acceptor is a dihydroxybenzene compound such as resorcinol. The in situ method has been found to be particularly effective where the reinforcing material is steel wire since pretreatment of the wire with the RFL system has been observed to be largely ineffective.
Resorcinol is known to form a resin network within a rubbery polymer by reacting with various methylene donors. Unfortunately, the use of resorcinol has some inherent disadvantages. Resorcinol is not readily dispersed in rubber and in fact neither the resin, nor the resorcinol become chemically bound to the rubber. Additionally, resorcinol in its raw form is excessively volatile and is potentially toxic, thus posing a health hazard. Another disadvantage in using resorcinol is periodic market shortages of supply.
There have been numerous attempts to replace resorcinol, however, few if any have had much success. For example, in U.S. Pat. No. 4,605,696 there is disclosed a method for enhancing adhesion of rubber to reinforcing materials through the use of phenolic esters as the methylene acceptor. These phenolic esters are less volatile than resorcinol, but still offer no readily reactive site for chemically attaching the resin to the rubber.
Bismaleimides have been used in rubber as vulcanization agents and increase the stiffness of diene rubber. The maleimide tail is believed to react with the diene-activated radical sites on the polymer generated by chemical, mechanical or thermal means, and increase the crosslinking network in the rubber. Unfortunately, bismaleimides do not reinforce the rubber by the formation of a resin network within the rubber.
Therefore, there exists a need to find a suitable replacement for resorcinol in an in-situ resin system while concomitantly improving rubber/reinforcement interaction for increased adhesion and tear in rubber.